The present invention relates to the phenomenon known as a fibre fuse, and in particular a method and apparatus for limiting the damage caused by a fibre fuse.
Optical power levels in optical transmission systems are generally increasing. This is due to a number of factors.
For instance, optical transmission systems, including optical fibres and other optical devices such as polarisation mode dispersion compensation mechanisms and photonic switches, all have attenuation characteristics. Increasing the power of an optical signal provides a better signal to background noise ratio, and allows the signal to be transmitted longer distances over the optical transmission system before optical amplification is required. Advances in laser technology have ensured that higher powered layers are now more readily and cheaply available, thus allowing a cost effective implementation of high optical power signal generation.
Typical optical transmission systems simultaneously transmit data using a multitude of different wavelengths, each transmission channel having a separate wavelength of light for transmission of the respective optical signal. Increasingly, channels are becoming more closely packed together with regard to wavelength e.g. DWDM (Dense Wavelength Division Multiplexed) systems. Increasing the number of simultaneous optical transmissions at different wavelengths will pro rata increase the average optical power being carried by the transmission system.
Many optical systems utilise optical amplifiers comprising optical fibre. An example of this is a Raman amplifier i.e. an amplifier that utilises the Raman effect. Optical amplifiers of this type normally use relatively high power pump lasers for providing the optical power that is utilised to amplify the optical signal power. Current trends indicate it is increasingly likely that Raman amplifiers will be utilised in future telecommunications systems.
Experiments have indicated that high power optical signals propagating through fibres can induce an effect referred to as a xe2x80x9cfibre fusexe2x80x9d. The fibre fuse effect, also termed selfxe2x80x94propelled selfxe2x80x94focusing (SPSF), is a catastrophic damage mechanism. Electronics letters, Jan. 7, 1988, Vol. 24, No. 1, pages 47-48 by R Kashyap and K J Blow and Electronics letters Jan. 5, 1989, Vol. 25, No. 1, Pages 33-34 by D P Hand and T A Birks describe this phenomena in some detail and describe a fibre fuse damage circuitxe2x80x94breaker, and are incorporated herein by reference.
The fibre fuse effect is believed to be initiated by local heating of the fibre. This can lead to a runway thermal effect which, provided the laser power is sufficient, continues until the fibre core melts. A thermal shock wave is created (visible as a bright spot of sidexe2x80x94scattered light) that propagates back along the fibre towards the optical power source. This results in the fibre being permanently damaged and unable to guide light.
Propagation velocity is believed to be of the order of tens of meters per second. A fibre fuse occurring in a telecommunications system could be extremely damaging. The sidexe2x80x94scattered light could be dangerous to any onlookers. Additionally, in systems where optical fibre spans (i.e. typically the length between optical fibre amplifiers) are of the order of 80 kilometers, it will be appreciated that if the fibre fuse is not contained, it has the capacity to damage large lengths of optical fibre. This would require replacement of the damaged fibre. Further, fibre fuses could damage additional components attached to the optical fibre e.g. amplifiers, pump lasers.
It is therefore desirable to limit the damage caused by fire fuses.
The ITU (International Telecommunications Union) document G.664, xe2x80x9coptical safety procedures and requirements for optical transport systemsxe2x80x9d provides guidelines and requirements to provide optically safe working conditions. In particular, it describes an automatic laser shutdown procedure in the event of a cable (optical fibre) break, and requires that the optical power be reduced to a Hazard Level 3A within 3 seconds of the break. Such a time period will clearly allow several tens of meters of optical fibre and related ancillary components to suffer damage.
It is an object of the invention to provide an improved apparatus and method for limiting the damage caused by a fibre fuse.
In a first aspect, the present invention provides method of protecting an optical system from a fibre fuse comprising the steps of; detecting a predetermined signal external to the fibre core, said signal being indicative of the fibre condition; providing a control signal indicative of said detected fibre condition. Fibre fuses are known to damage the optical fibre. By detecting eh condition of the fibre it is hence possible to detect if an event such as a fibre fuse has occurred. If a fibre condition indicative of a potential fibre fuse having occurred is detected, then a control signal may be provided to the system in order that further action can be taken to limit the damage. Such a control signal could be a continuous signal indicative that the fibre condition has not changed until a fibre fuse occurs, or alternatively a signal that only occurs once a condition indicative of a fibre fuse has occurred.
Preferably, the predetermined signal comprises a temperature measurement. Increases in temperature are associated with fibre fuses, and this temperature increase could be detected directly e.g. by a thermocouple or other heat measurement device.
The predetermined signal can be radiation emitted due to the fibre fuse process. It has been observed that a fibre fuse propagating along an optical fibre can be seen as a very bright white spot travelling along the fibre. This bright spot is thought to be caused by heating of the fibre e.g. a thermal glow. Measurement of this light can thus provide an indication of a fibre fuse.
Alternatively, the predetermined signal is at least a fraction of the signal transmitted along the fibre under normal operating conditions. In normal operation, an optical signal typically propagates longitudinally along the fibre. This signal can comprise any one or more of pump light, data signal(s) or other wavelengths. A fibre fuse will scatter this light, which can then be detected.
Preferably, the method further comprising the step of transmitting said predetermined signal so as to be incident upon the fibre.
Preferably, the predetermined signal is detected after at least one of a selection from the group of reflection from, refraction through, and transmission through the fibre. Reflection from, refraction through or transmission through the fibre can each be utilised to provide a signal indicative of the fibre condition.
Preferably, the incident signal is modulated.
Preferably, the incident signal comprises electro magnetic radiation. Alternatively, other types of signal could be utilised e.g. a beam of electrons or ions.
Preferably, the electro magnetic radiation is light.
Preferably, said predetermined signal is detected external to the fibre. For example, the signal can be detected radiating from the outer cladding of an optical fibre.
Preferably, the method further comprising the step of limiting the optical power output of a component to a level sufficient to quench a fibre fuse if said control signal in indicative of a fibre fuse.
Preferably, the method is performed in less than 3 seconds.
In a further aspect, the present invention provides a method of protecting an optical system from a fibre fuse comprising the steps of providing a length of fibre incorporating a beam expander arranged to quench a fibre fuse.
Preferably, the beam expander is a selection from a group of GRIN (Graded Refractive Index) lens, a ball lens, a bioconical taper, a selfloc lens and a thermally expanded fibre core.
In another aspect, the present invention provides an apparatus for protecting an optical system from a fibre fuse comprising a detector suitable for location external to an optical fibre core and arranged for detecting a predetermined signal indicative of the fibre condition.
Preferably, said signal is radiation emitted due to the fibre fuse process.
Alternatively, the signal is at least a portion of the signal normally transmitted along the fibre.
Preferably, the apparatus further comprising a transmitter arranged to transmit said predetermined signal so as to be incident upon the fibre.
Preferably, said detector is located external to the fibre.
Preferably, the apparatus further comprising a control means arranged to provide a control signal indicative of the detected fibre condition.
In a further aspect, the present invention provides a telecommunications system comprising an optical fibre and an apparatus for protecting the system from a fibre fuse comprising a detector located external to the optical fibre, said detector being arranged for detecting a predetermined signal indicative of the fibre condition.
Preferably, the said optical fibre includes a beam expander arranged to quench a fibre fuse.